Apparently Jason 2,876 according to this great site:
http://www.routemaster.org.uk/pages/history
There were variations of this lovely bus, but I'll leave the explanation of that to our on-site expert on these things, Seth!
An excellent website - so there's little that I can add to this thread!
Perusal has however uncovered an interesting and previously unknown fact to this scribe:
The final drive ratios specified were identical to several of those offered on AEC's contemporary single-deck 'Reliance' bus chassis. As all the LT vehicles had spiral-bevel final drives, I'm assuming identical crown wheel/pinion assemblies incorporated within an RM-specific rear axle casing for mounting within the rear sub-frame.
The 4.08:1 ratio on the BEA-liveried buses would give a top road speed well over 70mph (in a Reliance), though as all RM engines were set at a relaxing 1800 max RPM; so the top speed would be significantly lower. This also explains the lower maximum BHP outputs. Outside of LT; the AEC AV590/Leyland O.600 engines were usually set at 2200/2400 max RPM; and shoved-out up to around 150 horses depending on application.
The Routemaster was conceived and built for London, and although it was assembled by Park Royal, I'm reliably informed that LT retained title to the body jigs.
With a relatively expensive initial purchase tag, the only provincial recipient was Northern General, who found that the RM's all-aluminium structure stood-up extremely well to the harsh winter road conditions in the north east.
The whole concept was repeated some years later with the (supposedly!) all-conquering Leyland National - a joint venture between British Leyland and the vast National Bus Company ... but
that's another story for another day!